Subject: Re: [PATCH] Make CONFIG_MIGRATION available for s390
From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
To: Christoph Lameter <cl@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, schwidefsky@de.ibm.com, heiko.carstens@de.ibm.com,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Yasunori Goto <y-goto@jp.fujitsu.com>,
       Dave Hansen <haveblue@us.ibm.com>, Andy Whitcroft <apw@shadowen.org>
In-Reply-To: <4872319B.9040809@linux-foundation.org>
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	 <4872319B.9040809@linux-foundation.org>
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Date: Mon, 07 Jul 2008 19:28:09 +0200
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On Mon, 2008-07-07 at 10:09 -0500, Christoph Lameter wrote:
> This will extend the number of pages that are migratable and lead to strange
> semantics in the NUMA case. There suddenly vma_is migratable will forbid hotplug
> to migrate certain pages. 
> 
> I think we need two functions:
> 
> vma_migratable()	General migratability
> 
> vma_policy_migratable()	Migratable under NUMA policies.

Nothing will change here for the NUMA case, this is all about making it
compile w/o NUMA and with MIGRATION. What new strange semantics do you mean?
BTW, the latest patch in this thread will not touch vma_migratable() anymore,
I haven't read your mail before, sorry.

> That wont work since the migrate function takes a nodemask! The point of
> the function is to move memory from node to node which is something that you
> *cannot* do in a non NUMA configuration. So leave this chunk out.

Right, but I noticed that this function definition was needed to make it
compile with MIGRATION and w/o NUMA, although it would never be called in
non-NUMA config.
A better solution would probably be to put migrate_vmas(), the only caller
of vm_ops->migrate(), inside '#ifdef CONFIG_NUMA', because it will only be
called from NUMA-only mm/mempolicy.c. Does that sound reasonable?

> Hmmm... Okay. I tried to make MIGRATION as independent of CONFIG_NUMA as possible so hopefully this will work.

Umm, it doesn't compile with MIGRATION and w/o NUMA, which was the reason
for this patch, because of the policy_zone reference in vma_migratable()
and the missing vm_ops->migrate() function.

Thanks,
Gerald

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